Such a gas mask has become known from WO 86/06643. According to this disclosure a fan aspirates surrounding air, which is purified by a suitable filter--particle or dust filter, or gas filter--and undesired harmful substances are thus removed. The filter is taken up in a filter housing, which also simultaneously contains the fan and the connections and actuation elements necessary for driving it. Depending on the filter used, different fan powers are required: a gas filter requires only a small flow, so that it will not be exhausted prematurely, and a particle filter requires a greater fan power due to its elevated flow resistance. One type of filter element is adapted to depress a switch so that the fan runs at one speed whereas the other type of filter element does not engage a switch such that the fan runs at the other speed.
Another gas mask has become known from U.S. Pat. No. 3,496,703 which consists essentially of a backpack-like gas processing unit, which is connected to a breathing mask connected to a protective helmet via a fan connection. The air to be inhaled is processed in the gas processing unit by first passing ambient air through various filters and humidifying it. The respiration air thus processed is fed to the user of the gas mask and breathing equipment via the respiration hose. The delivery of the ambient air through the filters, the humidifier, and the respiration hose to the user of the gas mask and breathing equipment is ensured by an electric fan, which can be operated optionally from an internal or external battery. If the external battery is used to supply the fan with energy, the fan output can be varied in two steps, namely, a fast fan step and a slow fan step.
It is disadvantageous in the prior-art gas mask and breathing equipment that the fan output can be changed only when an external source of energy is used to drive the fan, and that the fan output can be changed only manually. It happens in practice that, depending on the field of application and the working conditions, different filters must be inserted into the gas mask and breathing equipment. For example, particle filters are to be used if dust is released in the environment during work, or gas filters must be used if gaseous toxic substances can be expected to occur in the working area atmosphere. It may also happen that so-called combination filters must be used when both dust and gaseous toxic substances are to be suspected in the working environment.
Known gas masks can in fact be adapted with respect to their fan power to the respective filter utilized and their service lives thus permit operational times that are as long as possible, but the person who wears the device, now as well as before, is left in uncertainty with respect to whether the filter utilized can be inserted in a functionally safe manner or whether during use it has become nearly exhausted and cannot be inserted again. In many cases of gas filtering, a breakthrough of the filter is either not noticed or can only be noticed later. Thus, for example, odorless harmful gases generally may not be detected by the wearer of the device.